This application claims benefit of provisional application Ser. No. 60/055,109 filed Aug. 6, 1997.
1. Field of the Invention
The present invention relates generally to wireless telecommunications networks, and more particularly, to a scalable wireless network architecture.
2. Discussion of the Related Art
As a wireless telecommunications network grows, in order to support an increasing number of subscribers, the number of mobile switching centers (MSCs) must be increased. This results in decreased efficiency in the MSCs due to networking and mobility overhead and thus effectively limits the number of MSCs in the network. As more and more MSCs are placed in the wireless network, the gain in capacity starts to diminish. When this point is reached, subscriber demands cannot be adequately satisfied since there is too much overhead. The overhead disadvantageously reduces the amount of central processing unit (CPU) power which is available for call processing.
Currently, the introduction of a new mobile switching center (MSC) into a wireless network involves geographically partitioning a serving area of a market region into partitioned serving areas. The partitioned serving areas are each assigned to one mobile switching center (MSC). As the number of mobile switching centers (MSCs) increase within the market region, the serving areas gets smaller. In addition, there is an increased number of boundaries between MSCs. The increased number of boundaries means an increased mobility of subscribers between MSCs, which in turn leads to an increased consumption of real-time CPU processing power. That is, extra mobility starts to dominate a total work of the network. As a result, there is a disadvantageous reduction in network capacity.
FIG. 1 illustrates an exemplary market region 10 of a wireless telecommunications network. The market region 10 is divided into partitioned serving areas 12, each serving area assigned to an MSC as shown in the legend of FIG. 1. FIG. 2 illustrates another exemplary market region 20 of a wireless telecommunications network. The market region 20 is divided into partitioned serving areas 22(1-6), each serving area assigned to an MSC/VLR 24(1-6) as shown. The boundaries between adjacent MSCs are indicated by reference numeral 26(a-b), where (a-b) specifies the particular boundary as shown in FIG. 2.
Some wireless communications network operators divide the serving area of the highest running switch in two, thus creating two serving areas. The network operators then assigns a new switch to one of these areas, while the old switch remains associated with the other half. This potentially leads to very unoptimized networks since a random boundary is not likely to be a good one in terms of subscribers crossing the boundaries. Ad hoc partitioning methods take this one step further, trying to eliminate high mobility borders by observing the traffic in the network. Still the result is likely to be non-optimal in terms of balance and borders.
Other current methods which are used for network planning involve optimizing the boundaries between MSCs to reduce the mobility across the boundaries, while trying to balance CPU loads of the switches. This is done through theoretical modeling of the network and usage of heuristic optimization methods for a much better optimization. For a small number of switches, this gives acceptable results even though some capacity loss occurs. When a higher number of MSCs are necessary, however, mobility overhead inherently becomes very high and finding good boundaries becomes increasingly more difficult, often impossible and thus making it necessary to settle for a not so desirable solution. Reducing mobility is therefore desirable since mobility presents an extra, non-revenue generating type of work to be performed by the network, as opposed to call processing which is used to generate revenue.
Further in connection with the above and the high growth rate of wireless telecommunication networks, there is continuously the need to serve an ever increasing number of subscribers. As the capacity of the existing mobile switching centers (MSCs) are exhausted, there is a further need to introduce new MSCs into the network. A desirable goal would be to support the highest number of subscribers with the smallest possible number of MSCs. The most important problem, however, is to reduce the networking overhead that results from mobility of subscribers between the MSCs, wherein the networking overhead includes such things as location updates and handovers. In connection therewith, Inter-VLR (Visitor Location Registry) location updates and Inter-MSC handovers use CPU real time and therefore disadvantageously reduce an available computing power for handling call processing related tasks.
Current methods for establishing multi-MSC networks will now be discussed in further detail. A typical scenario is to start with a single MSC that can serve the initial subscribers in a particular service area of a market region. Eventually, given that the market is large enough and that the operator is successful, the number of subscribers will increase and the initial capacity will become exhausted. The exact exhaust point will depend upon the subscriber profile. At this point, a second MSC is then needed to serve the new and existing customers.
The market region is now divided into two geographical partitions and each MSC is given control over a respective partition. Each region then also becomes a unique VLR (visitor location register), often integrated with the serving MSC. Each MSC controls its own BSCs, BTSs and subscribers, location areas, etc. in its respective service area. In addition, each MSC is to serve each and every customer that is physically located in the respective area or partition at any given time.
Network system partitioning may be carried out in a number of ways. As stated above, when there are multiple MSCs, there is a need to define the boundaries of the serving areas for each switch or MSC. There are several ways of defining a partition, including trivial ones such as follows. In one instance, a network operator may divide the highest running switch's serving area in the middle into two parts, assign one partition to the new MSC, while the old one continues to serve the other half. In another instance, the network operator may divide the network or system so as to give each MSC switch equal coverage area. Equality may be viewed in terms of number of cells served, total traffic, area of coverage, etc. The major problem with these methods is that the boundary between the MSCs is a disadvantageous source of extra mobility work on the network as mobile users move across a boundary or boundaries. For example, if a boundary happens to be placed in a particularly high mobility area, what would have previously been an intra-BSC or an inter-BSC handover over the particular boundary will now become an inter-MSC handover. The inter-MSC handover is a much more costly operation from a network operation point of view. The same is valid for location updates. The extra load that the MSC has to handle due to these location updates and handovers can be excessive and actually diminish the total capacity of the overall network. Still further, the resulting CM loads of all the switches should be in close balance to eliminate potential bottlenecks in the network. The network capacity computed depends on the switch with the highest CPU load. When this particular switch (i.e., the switch with the highest CPU load) reaches it engineering limit, the capacity of the network is considered reached although other MSCs may have remaining capacity. This remaining capacity, no matter how large, is rendered unusable. If there is a large unbalance, the network has to be physically rearranged to make it balanced. Naturally, the highest CPU load should go down (i.e., be reduced) in the process for obtaining a capacity gain.
Minimizing the mobility load while balancing the loads of the MSCs can be modeled as an abstract graph partitioning problem. Heuristics can be used to partition the network. Using heuristics, given that a number of MSCs are needed, coverage areas can be determined in such a way as to maximize subscriber capacity. Once a graph representing call processing and mobility within the network is obtained, projection can be used for determining as to how many switches are needed to support a given number of subscribers. The resulting graph, however, is sensitive to errors in the data collection process. In addition, implementation of a resulting study takes a long time to complete because it involves repartitioning BSCs, rehoming of BTS and BSCs, reassignment of LAC codes. Due to long time and high effort, network operators generally do not exactly implement the proposals, which may result in unbalanced loads and reduced capacity.
As higher numbers of MSCs are employed, networking overhead increases. Two MSCs working together will not have a total networked capacity of twice a single MSC due to mobility overhead. In other words, every additional MSC will give diminishing returns. As previously mentioned, examples of such mobility overhead include inter-VLR location updates per subscriber and inter-MSC handovers per call. Therefore, there is a practical limit to the number of MSCs that can be placed into a network with meaningful gains, after which the extra overhead work will eliminate any potential gains provided by the new switch.
Various abbreviations and acronyms are used herein and shall be understood to have conventional meanings as are known in the telecommunications art, except as may be noted otherwise herein.
It would thus be desirable to provide a scalable wireless network architecture which overcomes the problems discussed herein above.